Modern radio receivers, for example, radio receivers intended for mobile use, such as car radios, permit automatic switching to an alternative transmission frequency which may be received better and on which the same program is transmitted. If an alternative transmission frequency is receivable at the receiver's location, a switchover is made if the reception of a radio transmission frequency currently being received deteriorates, e.g., due to increasing distance of the receiver's current location from the radio transmitter.
To perform the switchover, radio receivers may include a decoder for analysis of information transmitted over a received radio transmission frequency in addition to the actual audio broadcasting program. This information may be transmitted, for example, by the Radio Data System (RDS). This information may include, for example, lists of alternative frequencies (AF) over which the same program is transmitted, the transmission frequency currently being received, and program identification codes (PI), which permit unambiguous identification of a program transmitted on a received transmission frequency. A description of a Radio Data System is referred to, for example, in “Specifications of the Radio Data System for VHF/FM Broadcasting,” European Broadcasting Union, Brussels, March 1984.
Before the radio receiver may switch from a radio transmission frequency currently being received to an alternative transmission frequency, e.g., on the basis of AF information, first the reception quality of the alternative transmission frequency should be tested. To do so, the radio receiver is tuned from the current radio transmission frequency to the alternative transmission frequency and the reception quality of the alternative transmission frequency is then determined, e.g., on the basis of the reception field strength or, e.g., on the basis of the error rate of the radio data signal, or on the basis of other criteria. If the reception quality of the alternative transmission frequency is better than that of the transmission frequency originally set, the alternative transmission frequency may be retained as the current transmission frequency. Otherwise, the radio receiver is tuned back to the transmission frequency originally set and/or additional alternative transmission frequencies are checked.
When switching from the current transmission frequency to an alternative transmission frequency, the reception quality of the alternative transmission frequency may not be known, e.g., the alternative frequency may have severe interference and the program may only be receivable with a high noise level. Also, the playback of the audio broadcast program transmitted over the set transmission frequency may be interrupted while the reception quality of the alternative transmission frequency is being checked. For simple tests, e.g., of only the reception field strength of the alternative frequency, the alternative frequency need be set only very briefly, i.e., in the range of a few milliseconds, so that an interruption of playback of the audio broadcast program may hardly be perceptible by a listener. For more complex tests, however, e.g., of the error rate of the radio data signal or the PI code of the alternative frequency, prolonged switching to the alternative frequency with correspondingly long playback interruptions in the range of at least a few tens of milliseconds up to a few hundreds of milliseconds may be necessary, these interruptions resulting from the time required for synchronization of the receiver to the RDS data stream and for analysis of the same. These interruptions may be audible and perceived as annoying by the listener, if suitable masking measures are not performed.
Various approaches have been researched and implemented, permitting switching to alternative reception frequencies without these interruptions being noticed.
European Patent No. 0 497 115 describes a radio receiver, which has a memory device for continuously receiving an audio broadcast program signal being played back. In the case of an interruption in playback of the audio broadcast program due to temporarily switching to an alternative radio broadcast signal for testing purposes, this interruption is masked by input of stored signal sections obtained from the audio broadcast program played back directly prior to the playback interruption.
European Patent Application No. 0 459 360 describes an RDS radio receiver having two receiving parts, a first receiving part tuned to a current radio broadcast frequency and the audio broadcast program transmitted on this radio broadcast frequency being played back. The second receiving part is tuned successively to alternative transmission frequencies for determining their reception quality, and the alternative transmission frequencies are stored in a list where they are sorted according to their reception quality. By constantly updating this list, the alternative frequency having the best reception may be inaudibly switched to, if a change in reception frequency becomes necessary because of a deteriorating reception of the current reception frequency.
German Patent Application No. 197 01 042 describes a method of analyzing RDS signals of an alternative transmission frequency to a radio transmission frequency currently being received, in which the radio receiver is switched by a high-speed phase locked loop from the current frequency to the alternative frequency within the duration of a single bit of the RDS signal, a data sample is taken from the RDS data stream of the alternative frequency, and finally, the radio receiver is switched back to the current transmission frequency. By repeated temporary switching and bit sampling, a section of the RDS signal of the alternative transmission frequency, which is sufficient for analysis, is input. The brevity of switching to the alternative transmission frequency and of the playback interruption associated with it result in these interruptions being largely inaudible. Since the samples taken are only fractions of the duration of an RDS data bit, they are sensitive to interference due to disturbances in reception. Therefore, a plurality of sections may be input and a largely interference-free section of the RDS signal of the alternative transmission frequency may be obtained by averaging.
German Patent Application No. 197 39 396 describes a modification of this procedure in that, in the case of temporary switching to the alternative transmission frequency, a plurality of directly successive bits of the RDS signal are input. This results in increased reliability of the input RDS data at the expense of longer playback interruptions, as well as longer periods of time for input of a section of the RDS signal sufficient for analysis, because the lengthier switchings need not be performed in direct succession to not be perceived as interfering.